how to texture a head model using exsiting images?
Try this: https://www.youtube.com/watch?v=KoiYMx2xXGA

A few suggestions (since you are new): While in object mode on the Tools Shelf (open with T key) select  Smooth under Shading. I'm assuming you don't want flat shading for a head :).

For hair I recommend Particle System based solution, if the model isn't being created for a game.

If you use your picture for hair it will probably look like these old 3d models in Playstation 2 , remember the flatness?

But there are some techniques for hair that are pretty good although made with pictures.

For any questions, don't hesitate to ask.